Image sensor and pixel that has positive transfer gate voltage during integration period

ABSTRACT

A pixel and image sensor formed in accordance with the present invention has two modes of operation: a normal mode and a low light mode. The present invention switches from a normal to a low light mode based upon the amount of illumination on the image sensor. Once the level of illumination is determined, a decision is made by comparing the level of illumination to a threshold whether to operate in normal mode or low light mode. In low light mode, the reset transistor (for a 3T pixel) or the transfer transistor (for a 4T pixel) is biased positive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/966,137, filed on 15 Oct. 2004, and claims priority therefrom under35 U.S.C. §120. The priority application is currently pending.

TECHNICAL FIELD

The present invention relates to image sensors, and more particularly,to an image sensor that uses pixels that maintain a positive transfergate (or reset gate for 3T pixel) voltage during the photodiodeintegration period under low light conditions.

BACKGROUND

Image sensors have become ubiquitous. They are widely used in digitalstill cameras, cellular phones, security cameras, medical, automobiles,and other applications. The technology used to manufacture imagesensors, and in particular CMOS image sensors, has continued to advanceat great pace. For example, the demands of higher resolution and lowerpower consumption have encouraged the further miniaturization andintegration of the image sensor.

For image sensors, it is a constant challenge to limit “hot pixels”,i.e. those pixels that output a signal even when there is no incidentlight. The hot pixel problem is particularly exacerbated during longintegration periods (i.e. long exposure times). The hot pixel problem isclosely related to dark current. As will be described further below, inthe case of a four transistor (4T) pixel design, it is believed that theregion beneath the transfer gate of the transfer transistor is a primarysource of dark current.

One method to limit dark current is to apply a negative potential to thetransfer gate. If the potential of the transfer gate is held negative,holes will accumulate under the transfer gate. This quenches electrongeneration which substantially reduces dark current and hot pixeleffects. However, the application of a negative potential to thetransfer gate results in reduced anti-blooming performance. At negativepotential, the transfer gate is shut off completely. If there is abright source of light incident on any of the pixels of the imagingarray, then the generated photocharge on those pixels cannot spill overthe transfer gate onto the floating node and from there over the resettransistor onto the V_(dd) drain. Instead, this high level ofphotogenerated charge spills onto adjacent photodiodes and results in alarge white spot on the image (i.e. “blooming”).

If the potential on the transfer gate is held positive, on the otherhand, there is an easy path through the transfer transistor and to theV_(dd) drain, since the transfer transistor is partially on. However, inthis state, the well capacity of the photodiode is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a three transistor (3T) active pixel.

FIG. 2 is a schematic diagram of a four transistor (4T) active pixel.

FIG. 3 is a flow diagram illustrating the method of operation of the 4Tactive pixel of FIG. 2.

FIG. 4 shows an image sensor formed using the active pixels and methodsof the present invention.

FIG. 5 is a timing diagram for a transfer transistor (4T pixel) or resettransistor (3T pixel) operating in normal light conditions.

FIG. 6 is a timing diagram for a transfer transistor (4T pixel) or resettransistor (3T pixel) operating in low light conditions.

DETAILED DESCRIPTION

In the following description, numerous specific details are provided toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventioncan be practiced without one or more of the specific details, or withother methods, components, materials, etc. In other instances, wellknown structures, materials, or operations are not shown or described inorder to avoid obscuring aspects of the invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrase “in one embodiment” or “in an embodiment” invarious places throughout the specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 illustrates a CMOS active pixel that uses three transistors. Thisis known in the art as a 3T active pixel. A light sensing element 101outputs a signal that is used to modulate an amplification transistor105. The signal is “placed” at node A which is connected to the gate ofthe amplification transistor 105. Thus, the amplification transistor 105is in source-follower configuration. The light-sensing element 101 canbe one of a variety of devices, including without limitation,photogates, photodiodes, pinned photodiodes, partially pinnedphotodiodes, etc.

During an integration period (also referred to as an exposure oraccumulation period), the light-sensing element 101 captures light thatis incident onto the pixel and outputs a signal indicative of the amountof light incident on that light-sensing element 101. The signal (whichis an accumulation of charge generated by the light sensing element 101)is stored on node A which is connected to the gate of the amplificationtransistor 105. The signal stored on node A is then used to modulate theamplification transistor 105. After the integration period, a resettransistor 103 is used to reset the level of the node A to a referencelevel prior to the start of the next integration period. Finally, a rowselect transistor 107 is used as a means to address the pixel and toselectively read out the signal onto a column bit line 109.

FIG. 2 is similar in many respects to the 3T active pixel of FIG. 1,except that an additional transfer transistor 201 is used to transferthe signal output by the light-sensing element 101 to a floating node B.This design is known as a four transistor or 4T pixel. In operation,during the integration period, the light sensing element 101 generatescharge which is held at the light sensing element because transfertransistor 201 is off. After the integration period, the transfertransistor 201 is turned on (using the transfer gate) to transfer thesignal to the floating node B. After the signal has been transferred tofloating node B, the transfer transistor 201 is turned off again for thestart of a subsequent integration period. Thus, as seen, the transfertransistor 201 turns on and off periodically to transfer signal fromeach integration period to the floating node B.

The signal on the floating node B is then used to modulate theamplification transistor 105. Further, after readout through the columnbitline 109, the reset transistor 103 resets the floating node B to areference voltage, in this particular embodiment, V_(dd).

The present invention is equally applicable to either of the 3T or 4Tdesigns described above, as well as the 5T, 6T, and other designs usedby solid state image sensors. Specifically, the present inventioncontemplates that there are two modes of operation for the pixel (andthus the image sensor). Under low light conditions, the voltage on thetransfer gate (in the case of a 4T, 5T, 6T and any other pixel designsthat use a transfer gate) is held slightly positive. Under normal lightconditions, the pixel operates normally with the voltage on the transfergate nominally at about 0.0 volts. Similarly, for the 3T pixel, underlow light conditions, the voltage on the reset gate of the resettransistor 103 is held slightly positive. Under normal light conditions,the pixel operates normally with the voltage on the reset gate nominallyat 0.0 volts.

The structure of an active pixel formed in accordance with the presentinvention is substantially the same as that shown in FIG. 1 or 2.However, as further detailed below, the operation of the image sensorand the active pixels within the image sensor is controlled such thatthe pixels behave differently based upon different light conditions.

Turning to FIG. 3, a flow diagram showing a method of the presentinvention is provided. First, at box 301, the illumination levelincident to the image sensor (and thus pixels) is monitored. This can bedone in any number of conventional ways. For example, the output fromthe image sensor can be examined for its brightness level. As can beappreciated of ordinary skill in the art, nearly every image sensor hascircuitry for automatic gain control and exposure control. Bydetermining the strength of the signal output from the pixels, theambient light level can be determined. Alternatively, the processedoutput from the image sensor can be examined to determine the relativeambient lighting conditions. Still alternatively, a dedicatedlight-sensitive device outside of the imaging area of the image sensorcan be used to monitor the amount of incident light onto the imagesensor.

Next, at box 303, the illumination level determined at box 301 iscompared to a threshold value, such as in one embodiment by the use of acomparator circuit. The threshold value is the trigger for transitionbetween “normal” operation and “low light” operation. The precise pointwhere the threshold is set may be made variable depending upon designconsiderations, parameters, and characteristics of the image sensor, andmay even be adjustable at the discretion of the user of the imagesensor. In any event, in one embodiment, a threshold is used todelineate between normal and low light operation. It can be appreciatedthat other types of triggers may be used to transition from normal andlow light operation. The precise mechanism used to transition fromnormal to low light operation is not critical.

If the illumination level determined at box 301 is lower than thethreshold, then at box 307, the transfer transistor (or reset transistorin the case of a 3T pixel) is positively biased to some level V_(tx)_(—) _(lo) _(—) _(dark) during the integration period. In oneembodiment, V_(tx) _(—) _(lo) _(—) _(dark) is 0.5 volts. However, theexact value of V_(tx) _(—) _(lo) _(—) _(dark) will vary with theparticular process (e.g. 90 nm CMOS) used to manufacture the imagesensor and may vary from just above 0.0 volts and just under full turnon of the transistor.

In the case of a 4T pixel, during the readout period that transfers thephotodiode signal to the floating node, the transfer transistor isbiased to V_(tx) _(—) _(hi), which will fully turn on the transfertransistor. In some embodiments, V_(tx) _(—) _(hi) may be 1.3 volts, 1.8volts, 2.3 volts, 2.8 volts, or 3.3 volts. In other words, V_(tx) _(—)_(hi) is the turn on voltage of the transistors in the integratedcircuit and the precise voltage will vary with the particular integratedcircuit. Typically, V_(tx) _(—) _(hi) is simply the V_(dd) power supplyrail for the integrated circuit. In other embodiments, V_(tx) _(—) _(hi)may even be higher than the V_(dd) power supply rail if a bootstrappingor charge pump circuit is used. FIG. 6 illustrates an example of thecontrol timing for the transfer transistor. Specifically, during anintegration period 601, the transfer transistor is held at V_(tx) _(—)_(lo) _(—) _(dark). Then, during the readout periods 603, the transfertransistor is biased to V_(tx) _(—) _(hi). Note that in the case of a 3Tpixel, the above described timing is applied to the reset transistor103.

However, if the illumination level is higher than the threshold, then atbox 305, the pixels operate normally with V_(tx) _(—) _(lo) _(—) _(norm)at or near 0.0 volts. Thus, the transfer transistor (or reset transistorin the case of a 3T pixel) is held at V_(tx) _(—) _(lo) _(—) _(norm)during the integration period. In one embodiment, V_(tx) _(—) _(lo) _(—)_(norm) is 0.0 volts.

In the case of a 4T pixel, during the readout period that transfers thephotodiode signal to the floating node, the transfer transistor isbiased to V_(tx) _(—) _(hi), which will fully turn on the transfertransistor. FIG. 5 illustrates an example of the timing for the transfertransistor. Specifically, during an integration period 501, the transfertransistor is held at V_(tx) _(—) _(lo) _(—) _(norm). Then, during thereadout periods 503, the transfer transistor is biased to V_(tx) _(—)_(hi). Note that in the case of a 3T pixel, the above described timingis applied to the reset transistor 103.

Furthermore, it can be appreciated that the precise values of V_(tx)_(—) _(lo) _(—) _(dark), V_(tx) _(—) _(lo) _(—) _(norm), and V_(tx) _(—)_(hi) may vary with the particular device and system characteristics, aswell as with the process used to manufacture the image sensor. However,V_(tx) _(—) _(lo) _(—) _(dark) is typically greater than V_(tx) _(—)_(lo) _(—) _(norm).

To understand why the above timing and voltage bias combinations areadvantageous, further analysis of the operation of the pixel isprovided. Specifically, if the ambient lighting is normal (or non-lowlight), the gain of the image sensor typically remains at 1.0. Underthese conditions, dark current and hot pixel generation is not asignificant issue since the integration periods are relatively short.

However, in low light conditions where the image sensor gain isincreased, for example to 1.5, blooming is improved since the transfertransistor or reset transistor (for 3T pixels) is partially on.Moreover, the full well capacity is not degraded since the gain isgreater than 1.0 and the image sensor does not use the full wellcapacity of the photodiode. For example, if the gain is 2.0, then theimage sensor can only effectively use half of the full well capacity ofthe photodiode due to signal swing limitations at the higher gain. Thus,the fact that the transfer transistor or reset transistor (for 3Tpixels) is partially “on” during integration at gains greater than 1.0does not degrade the signal to noise ratio of the image sensor.

Dark current and hot pixel defect density is also improved. It has beenfound that the increased generation of electrons under the transfer gateas a result of the positive bias during low light conditions results inbetter performance. Because the transfer transistor or reset transistor(3T pixels) is “on”, the generated charge under the transfer gate flowsto the floating node. As part of the readout timing, the floating nodeis reset and all of this generated charge is swept to the V_(dd) throughthe reset transistor 103 when the reset transistor 103 is turned on toreset the floating node potential. In this manner, dark current and hotpixel defect density is improved. In the case of a 3T pixel, the resettransistor is partially on which allows the generated electron charge togo out to V_(dd).

Thus, as seen from the description above, a pixel and image sensorformed in accordance with the present invention has two modes ofoperation: a normal mode and a low light mode. The present inventionswitches from a normal to a low light mode based upon the amount ofillumination on the image sensor. The amount of illumination on theimage sensor can be determined in a variety of ways, and any method fordetermining the level of illumination could easily be applied to thepresent invention. Once the level of illumination is determined, adecision is made by comparing the level of illumination to a thresholdwhether to operate in normal mode or low light mode. In low light mode,the reset transistor (for a 3T pixel) or the transfer transistor (for a4T pixel) is biased positive.

The active pixels described above may be used in a sensor array of aCMOS image sensor 1101. Specifically, FIG. 4 shows a CMOS image sensorformed in accordance with the present invention. The CMOS image sensorincludes a sensor array 1103, a processor circuit 1105, an input/output(I/O) 1107, memory 1109, and bus 1111. Preferably, each of thesecomponents is formed on a single silicon substrate and manufactured tobe integrated onto a single chip using standard CMOS processes.

The sensor array 1103 portion may be, for example, substantially similarto the sensor arrays portions of image sensors manufactured by theassignee of the present invention, OmniVision Technologies, Inc., ofSunnyvale, Calif., as model numbers OV5610 or OV7640, except that thepixels are replaced with the active pixels disclosed herein.

The description of the invention in this application as set forth hereinis illustrative and is not intended to limit the scope of the invention.Variations and modifications of the embodiments described herein arepossible, and practical alternatives to, or equivalents of the variouselements, the embodiments are known to those of ordinary skill in theart. These and other variations and modifications of the embodimentsdisclosed herein may be made without departing from the scope and spiritof the invention.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. For example, the teachings of thepresent invention can be applied to a 5T, 6T, 7T, or other types ofpixel architectures. Accordingly, the invention is not limited except asby the appended claims.

1. An active pixel comprising: a light sensing element formed in asemiconductor substrate and connected to a node; and a reset transistorconnected between said node and a reference voltage and selectivelyoperable to reset said node to said reference voltage, wherein saidreset transistor is positively biased during an integration periodduring a first mode of operation that is selected if a level of incidentlight is low, and wherein said reset transistor is turned off duringsaid integration period during a second mode of operation that isselected if a level of incident light is normal.
 2. The pixel of claim 1wherein said light sensing element is selected from the group ofphotodiode, pinned photodiode, partially pinned photodiode, orphotogate.
 3. The pixel of claim 1 further including an amplificationtransistor controlled by said node, wherein said amplificationtransistor outputs an amplified version of said signal to a columnbitline.
 4. The pixel of claim 1 wherein said pixel is integrated into aCMOS image sensor.
 5. A method of operating a pixel of an image sensor,said pixel including a light sensing element that is connected to anode, a reset transistor between said node and a reference voltage, andan amplification transistor modulated by said signal on said node, themethod comprising: determining a level of incident light; if said levelof illumination is low: partially turning on said reset transistorduring an integration period; and if said level of illumination isnormal: substantially turning off said reset transistor during saidintegration period.
 6. The method of claim 5 wherein said light sensingelement is selected from the group of photodiode, pinned photodiode,partially pinned photodiode, or photogate.
 7. The method of claim 5wherein said amplification transistor outputs an amplified version ofsaid signal to a column bitline.